EP544 - Mucinous Adenocarcinoma Arising in a Chronic Perianal Fistula in Crohn’s Disease: A Rare Case Report

Purpose/Background: Crohn's disease (CD) is frequently accompanied by anorectal disease with perianal fistulas being one of the most common manifestations. These fistulas are usually benign processes, however, malignancy is reported in chronic non-healing fistulas. Mucinous adenocarcinoma arising in perianal fistulas in CD is a rare condition with the incidence rate of 0.7%. It originates from the epithelial lining of the perianal fistula internally rather than the rectal mucosa. Timely diagnosis is the biggest challenge leading to treatment delays. Standard treatment consists of neoadjuvant chemoradiation followed by surgical resection. We present a rare case of mucinous adenocarcinoma arising in a chronic perianal fistula in a patient with CD.

Methods/Interventions: A 47-year-old male with 18-years history of CD involving terminal ileum and complex perianal fistulas presented with worsening rectal pain and perianal drainage. Physical exam and magnetic resonance imaging (MRI) pelvis were consistent with complex right-sided perianal fistulas with multiple skin openings extending to the right gluteus. He underwent an exam under anesthesia, Seton placement, and tissue biopsies. Pathology showed granulation tissue and a small focus of indeterminate dysplasia. Despite optimizing treatment with biologic agent and adequate control of luminal disease, he had worsening anorectal symptoms and underwent repeat biopsies. Pathology showed mucin-producing adenocarcinoma of colorectal origin. MRI pelvis showed a 13.6 x 7.5 x 6.2 cm mass arising in the complex anal fistulas without rectal mass (Figure).

Results/Outcomes: He completed total neoadjuvant chemotherapy followed by robot-assisted abdominoperineal resection with en-bloc resection of right gluteal lesion. Final Pathology showed moderately differentiated mucinous adenocarcinoma in the gluteal lesion (ypT3N0). He underwent delayed perineal wound closure with flap reconstruction and has recovered well.

Conclusion/Discussion: Perianal fistula in CD is often complex. Chronic inflammation, fibrosis, and recurrent infections may mask early malignant changes and delay the diagnosis. This case highlights the challenges and importance of timely diagnosis of adenocarcinoma in perianal fistula. Persistent, indurated, or atypical fistulas should prompt early and repeat biopsy of deeper tissue and multidisciplinary evaluation. Vigilant surveillance remains critical to improve early diagnosis and patient outcomes.

Figure
13.6 x 7.5 x 6.2 cm large markedly T2 hyperintense mass extending from the distal anal canal and inferiorly through the anal verge to occupy large portion of the right gluteal cleft.